(a) Field of the Invention
This invention relates to a needle-carrying head for a printing machine.
(b) Description of the Prior Art
In known printing machines using needles, in which the needles are operated by means of electromagnets in such a way that their front ends strike an inking ribbon and thus produce individual printed points on the printing support, a bank of the needles is situated in a plane perpendicular to the writing direction, that is to say parallel to the direction of the needle spacing.
Each printed character is thus made up of a succession of points, and not of continuous lines. It is accordingly necessary, in order that the writing be legible on the one hand and so that its appearance be aesthetically acceptable on the other hand, that the printed points are of very small diameter (not greater than the width of the lines for which they are substituted) and, especially, that their definition be very high, that is to say that they are very close to each other.
This double condition gives rise to very difficult problems relating to the construction of the machine. As a matter of fact, if nine needles of a diameter of about some hundredths of millimeters (30/100 millimeter, for instance) must be juxtaposed within the height of a character, of 3 mm for instance, the distance separating the needles will be slightly more than 3/100 millimeter. Now, it is impossible to juxtapose mechanical means for driving needles in so small a distance. This necessitates, in the known needle-carrying heads, that the needles are bent so that their rear ends are considerably more widely spaced (for instance 4 mm) than their front ends.
This arrangement results in a construction which is relatively complicated, needles which are relatively long and, by reason of their deflection, to the need for a precise guiding of the needles which, however, does not prevent the risk of buckling being increased. Finally, the length of the needles, determined by their deflection and which is relatively important, results in the fact that the mass of the needles is not negligable and this restricts the striking speed.